tests: head: avoid false failure on some systems

* tests/head/head-c.sh: Pass a more similar operation
to get_min_ulimit_v_, so we get a more appropriate limit.
This was seen to be significant with CheriBSD.

tests/head/head-c.sh

@@ -39,7 +39,7 @@ esac
 # Only allocate memory as needed.
 # Coreutils <= 8.21 would allocate memory up front
 # based on the value passed to -c
-vm=$(get_min_ulimit_v_ head -c1 /dev/null) && {
+vm=$(get_min_ulimit_v_ head -c-1 /dev/null) && {
   (ulimit -v $(($vm+8000)) && head --bytes=-$SSIZE_MAX < /dev/null) || fail=1
   (ulimit -v $(($vm+8000)) &&
     head --bytes=-${UINTMAX_OFLOW}000 < /dev/null) || fail=1